1. Field of the Invention
The present invention relates to an amplifier circuit and a method of signal amplification. More particularly, the present invention relates to signal amplification without using capacitors.
2. Description of the Related Art
FIG. 1 is a schematic diagram showing a conventional amplifier circuit 100 which drives the speakers 121 and 122 of a stereo headphone. The amplifier circuit 100 includes two operational amplifiers (OPAs) 101 and 102. The OPA 101 amplifies the left channel input signal 151 and then outputs the amplified input signal to the left channel speaker 121. The OPA 102 amplifies the right channel input signal 152 and then outputs the amplified input signal to the right channel speaker 122. The power supply voltage of the OPAs 101 and 102 is VDD. The voltage ½ VDD coupled to the positive input terminals of the OPAs 101 and 102 is the reference voltage of the amplification of the input signals 151 and 152. The speakers 121 and 122 are coupled to the 0V ground voltage, while the OPAs 101 and 102 use the voltage ½ VDD as the reference voltage of signal amplification. Due to the difference of voltage levels, the serial capacitors C1 and C2 are required for rejecting DC current.
Unfortunately, the serial capacitors C1 and C2 cause some disadvantages. Firstly, the capacitors C1 and C2 have an adverse impact on the frequency response in the lower frequency band. The high pass filter formed by the capacitors C1 and C2 rejects voices passing through it. Secondly, the capacitors C1 and C2 increase the area and the cost of the circuit layout. In addition, the aforementioned difference of voltage levels charges the capacitors C1 and C2 and produces a pop noise when the amplifier circuit 100 is enabled suddenly. For example, when the user turns on the power of the amplifier circuit 100, the pop noise may be produced and output through the stereo headphone, which is annoying.